The present invention relates to an arrangement for an instrument or such an instrument which comprises or is connected to a screwing-in or tightening tool, referred to here as a driving-in tool, for elements on or in the form of an implant or drill for bone, in particular dentine. In this connection, the tool comprises an electric motor which can be controlled by a control unit. Also included is a unit which senses feed current in the electric motor and, depending on the sensing, emits a signal depending on the torque of the electric motor, by means of which signal a torque or torque-angle curve can be represented. Also included is a display unit which is arranged so as to display fully or partly a driving-in or tightening function performed. Thus the display unit can display, for example, curves or collected or individual values.
The present invention constitutes a development of inter alia the equipment indicated in PCT publication WO 98/27886, which relates to a surgical instrument to which a tightening tool according to the above can be connected and which instrument includes a display unit in order to display inter alia a torque-angle value. The instrument is used inter alia to screw implants or fixtures tight in the dentine. The instrument can also be used to screw down a screw tap in a hole made in the dentine. Furthermore, the instrument can be used to screw elements in the form of screws tight in association with said implants and spacer elements belonging to these and other prosthetic constructions. The known instrument comprises a control unit for controlling the electric motor and also sensing means which sense the current in the feed circuit of the motor and, depending on the sensing, supply information to microprocessor-based equipment.
The present invention is also based on what is previously known through PCT publication WO 95/20146 which indicates equipment for inter alia screw-driving and in which the structure in question must fit against the interacting structure with a high degree of accuracy. One of the main objects of this invention is that there are not to be any residual stresses or strains which are built into the structure as a result of assembly.
There is a requirement to be able to improve further the previously known instruments. In this respect, it is desirable in practice, for example, to be able to perform practical analysis aimed directly at the quality of the driving-home or tightening function in screw joints. The invention solves this problem inter alia.
It is also desirable to have a simple and readily understandable indication of the quality of the tightening function in question. The invention solves this problem also.
In connection with the driving-home of screws in implants or equivalent, the screwing-in function can be impaired by bone penetrating, for example, between fixture and spacer. Another source of defects may be that the spacer in question is not sufficiently well fitted or anchored on the implant. A further factor which can distort the driving-home function is if the entire prosthetic function as such is poorly aligned. There is a requirement for sources of defects of said type not to be capable of impairing the driving-home result, but for it to be possible to detect them at an early stage during the installation work. The invention solves this problem also.
There is a requirement to be able to determine and check a desired or predetermined anchoring load in the element or screw concerned or the securing force in the screw connection. The invention solves this problem also.
There is in this respect a requirement to be able to identify the frictional resistance the element or screw encounters in the screwing-in function concerned. The invention solves this problem also.
There is also a requirement to be able to evaluate or assess the bone quality during ongoing implant preparation. According to previously known instruments, the bone hardness can be evaluated during preparation in connection with threading of a prepared hole using a screw tap or a self-tapping fixture. The disadvantage of previously known methods is that the surgeon has an opportunity to establish the bone quality only after the hole has been drilled. It would be a great advantage if the surgeon could know the bone hardness before making a decision on which drill diameter to use for the final preparation. The invention aims to solve this problem inter alia.
There is also a requirement to be able to establish, during the driving-in function, whether the implant is rotating without following the pitch of the implant in the axial direction, which means that the threads are being destroyed in the bone. The invention aims to solve this problem also.
An arrangement of the type referred to in the introduction can be considered to be characterized mainly in that the sensing and/or display unit determines the quality of the tightening function by means of a scale based on the slope of the torque-angle curve produced. According to the invention, a flatter slope of the curve represents lower quality and a steeper slope represents high quality.
The invention can also be considered to be characterized mainly in that a calculating unit, for example in the sensing and/or display unit, is arranged so as to determine, in the tightening function, a frictional resistance which exists for the element during its driving-in or screwing-home in order to make possible the application of an accurate desired or predetermined loading which brings about secure locking (preload) of the element by means of tension or elasticity movement in the material of the element.
In a development of the inventive idea, the element consists of a screw associated with the implant/the fixture and the spacer. The display unit can in this connection be arranged so as to display said scale, for example a scale running between 1 and 10, where 10 indicates good quality and 1 indicates poor or inferior quality. The slope can be defined with a starting point from a base value of the torque. The slope can also be defined from a base value up to the maximum torque value in question and also by means of the range of degrees of rotation for the sloping part concerned of the curve.
Said developments can also relate to a screw which is first intended to be screwable home with frictional resistance inter alia via its screw thread and can then be anchored by means of tension or elasticity in the screw material (preload) which is required for the anchoring loading on the screw. Said calculating unit can operate with a torque value which constitutes part of a set torque value and brings about tightening of the element or screw. In this connection, a first measured value can be established. The calculating unit can also establish a second measured value when the element or screw is subsequently loosened. Taking as its starting point said first and second value, the calculating unit can work out a frictional resistance according to a formula indicated below. The calculating unit can then work out the torque value in question by means of adding to the frictional resistance worked out a value which gives the desired or predetermined secure-locking loading (preload) on the element.
An arrangement for evaluating the bone quality during implant preparation can be considered to be characterized mainly in that the sensing unit senses torque generated by the electric motor during drilling in the bone, in particular dentine, of a hole for a screw tap or a threaded implant, and in that the display unit indicates, depending on the sensing, a torque-time curve, by means of which the quality of the bone can be read off during drilling.
In developments of the inventive idea, the sensing unit senses the torque during drilling and the calculating unit is then arranged so as to calculate power consumption per millimeter of drilled hole/implant. Empirical evaluations can also be included as a component in the quality read-out. A curve displayed on the display unit indicates the average power per millimeter of implant. The drill speed can also be included in the calculation.
The invention is further characterized in that the sensing unit senses torque generated by the electric motor during tapping of the implant into the bone, preferably the dentine, and in that a calculating unit, for example in the sensing or display unit, is arranged so as to establish, during the driving-in function, whether the implant is rotating without following the pitch of the implant in the axial direction, that is to say the threads are being destroyed in the bone, and also in that said arrangement warns the user that this is taking place or stops the rotation of the implant.
By means of the proposals made above, previously known instruments can be supplemented with new functions which are of value in being able to perform the practical work. The possibility of integration into known instruments has economic advantages. Furthermore, it is possible to prevent stresses being built into the implant constructions with implants, spacers etc. The torque-speed curve in question during drilling can give the user sufficient information for interpreting his/her own actions because he/she knows his/her own drilling style and the arrangement can therefore be an extremely valuable diagnostic tool. As the user also establishes more and more references in the actions he/she takes, the accuracy of interpretation will increase.